1. Field of the Invention
The present invention relates to an accelerometer-type sensor and, more specifically, to a sensor of this kind which is sensitive to the phenomena of pinging liable to affect the operation of an internal combustion engine.
FIG. 1 of the appended drawing is a diagrammatic depiction, in an exploded view, of a known pinging sensor of the prior art, comprising a metallic cylindrical base 1 of axis X formed of an axial barrel 2 protruding from an annular shoulder 3, the barrel and the shoulder both having an axial hole 4 passing through them, the sensor further comprising a number of washers slipped over the barrel 2 and stacked on the shoulder 3. Thus, starting from this shoulder and stacked in this order, there are: a washer 5 made of an electrically insulating material, a washer 6 made of a piezoelectric material and its electrodes 7.sub.1, 7.sub.2, each one backing onto one of its axially-separated end faces, a second insulating washer 8, a weighty washer 9, a spring washer 10 and a metal nut 11 screwed onto a screw thread 12 formed on the barrel 2, the nut clamping all of the above listed washers down against the shoulder 3 of the base.
The sensor described above can be fixed to an internal combustion engine using a bolt passed through the axial hole 4. It is known that such a sensor is sensitive to the forces experienced by the engine along the X-axis, such as vibrations due to the pinging phenomenon, and delivers, between the electrodes 7.sub.1, 7.sub.2, an electrical signal which represents these forces, the signal being the result of the pressure exerted by the mass 9, accelerated by said forces, on the piezoelectric washer 6. It is also known that when pinging is detected in one cylinder of an internal combustion engine, the computer controlling the operation of the engine reacts by reducing the angle of ignition advance in an attempt to cause this pinging to disappear, as the pinging could otherwise, in the long term, be responsible for mechanical damage to the engine.
The use of a nut to hold the various washers of the sensor on the barrel of the base has numerous drawbacks. On the one hand, the tightening of the nut entails the use of a torque-measuring tightening head to establish a predetermined preload on the piezoelectric washer. When such a sensor is being mass-produced, the preload thus created may vary with wear to the head and with variations to the geometry of the tapped thread of the nut or the screw thread of the barrel, which may, in particular, be corrupted by impacts. Furthermore, swarf may form while the screw thread and/or the tapped thread are being machined. When the sensor is being mounted, swarf 14 thus formed, held in the screw thread of the barrel or in the tapped thread of the nut, may become detached and find its way in between the electrodes 7.sub.1, 7.sub.2 of the piezoelectric washer 6, thus short-circuiting these electrodes and making the sensor unusable.
The use of a nut in such a sensor also has the drawbacks of being expensive and of complicating the automating of the assembling of the sensor.
2. Description of the Related Art
There is known, from German Patent No. 195 24 152, a pinging sensor in which the various washers are not retained by a nut but by five lugs formed by punches acting on the exterior of the surface of the metal barrel to tear the material of these lugs from this barrel and upset it outward over the elastic washer 10, whose axial position on the barrel is thus fixed.
This solution does, however, have the following drawbacks. On the one hand, punching of this kind may generate metallic swarf and therefore the short-circuits mentioned earlier. On the other hand, in the context of mass-production, the preload on the piezoelectric washer can vary with variations in the punching forces, which depend on the condition of the cutting edges of the punches, how sharp they are, etc, etc.